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Organics Questions

Definitions of Volatiles and SemivolatilesTest Methods:  Frequently Asked Questions

Question: Is there an official EPA definition of a volatile compound and a semivolatile compound?

Answer: SW-846 does not contain formal definitions for volatiles or semivolatiles and there is no "official" definition of volatile or semivolatile compounds that can be used across all EPA programs.

Some compounds can even be considered either a volatile or a semivolatile depending on the analytical technique used (i.e., naphthalene, nitrobenzene, dichlorobenzene, hexachloroethane). Generally speaking, volatiles have been defined elsewhere as compounds with boiling points below 150C and vapor pressures of greater than 0.1 mm Hg. However, such definitions are not hard and fast, and should only be considered descriptive of a general trend. In fact, Method 8260B contains a few compounds that are exceptions to this definition.

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Calibration and Swapping GC Columns

Question: Sec. of Method 8000B states that "some maintenance procedures are so likely to affect the instrument response that recalibration is automatically required." I have a hypothetical situation. I am running Method 8081A using a J&W DB-5 column and I must confirm a positive result. Therefore, I remove that column and replace it with a J&W DB-608 column and perform my confirmation analysis. I then go back to the DB-5 column and analyze a calibration verification standard which meets all criteria. Why should I re-calibrate?

Answer: You must recalibrate because you cannot guarantee that every fitting was swaged to the same degree, that the column is placed in the injector and detector exactly as it was before, that the flow rate through the column is exactly as it was before, that you did anything other than a one-point calibration, or that you have not created active sites in the GC system. Simply passing the acceptance criteria for the calibration verification is not sufficient evidence that the rest of the calibration was not affected by swapping the columns in and out.

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Method 3620 Florisil Cartridge Check

Question: We are using the Florisil cartridges described in Method 3620B and we cannot find any that meet the performance checks described in Sec. 8 of the method. Specifically, we cannot get the recovery of trichlorophenol to be less than 5%.

Answer: While the use of the Florisil cartridges may require a bit of practice, the root cause of your problem may be the result of an editorial error in the "B" version of this method. At some point during the development of Method 3620B, the units for the cartridge check solution in Sec. 5.11 were changed to milligrams per liter when the units should be micrograms per liter (mg/L versus ug/L). As a result, the cartridge check solution is 1000 times more concentrated than intended and may overload the cartridges, allowing the trichlorophenol to break through. This editorial error was corrected in the latest version of the method, 3620C, which is available in Update IVB.

However, at that time, a new error was introduced in Sec. 5.10, when the units for the trichlorophenol solution were similarly changed and should not have been. The correct concentration for the solution in Sec. 5.10 is still 0.1 mg/L, or 100 g/L.

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Purge Times for Methods 5030, 8015, and 8260

Question: Method 5030B specifies a purge time of 11 minutes when using Method 8260 and 15 minutes for Method 8015. Can we change these purge times, or are they cast in stone?

Answer: Yes, the purge times can be changed from those listed in the method. The purge times in Method 5030B (and other sample preparation methods) are recommendations, based on the work performed in developing the original procedures. Under the flexibility inherent in most SW-846 methods, you are free to modify the conditions to meet the objectives of a given analysis. Therefore, if you can demonstrate that other purge times perform well enough for your application, then you are free to use them, provided that you use the same purge time for the calibration associated with the samples. EPA plans to clarify this in the next revision of Method 5030, as it has in other recent methods.

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Method 5035 and Holding Time for the EnCore Sampler

Question: What is the current holding time for samples collected in the EnCore sampler and analyzed using Method 5035?

Answer: Between the time that Method 5035 was proposed and when it was promulgated, EPA reviewed data from the manufacturer of the EnCore device. Those data suggested that samples could be held in the EnCore device for 48 hours without significant loss of volatile analytes. Subsequent data for freezing samples in the EnCore sampler also indicates a 48-hour limit without significant loss. Therefore, as noted in Sec. of the promulgated version of Method 5035, samples may be held for up to 48 hours before analysis, or before transfer to the glass vials used for the closed-system purge-and-trap device in the method. The overall holding time for the sample remains 14 days from collection to analysis. The same considerations apply to Method 5021 and the EnCore sampler.

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Method 8151A Calibration Standards

Question: Sec. 5.12 of Method 8151A states that standards may be purchased as certified solutions from commercial vendors. Does this mean that we can use commercially-available standards of the methyl esters?

Answer: No, except in very specific circumstances. Method 8151A involves the derivatization of the target herbicides in the sample extracts. As a result, the calibration standards undergo the same derivatization process as the samples (although it is not necessary to derivatize them in the same batch as the samples). Therefore, if the samples are to be derivatized, then the standards must be prepared from the target herbicides and derivatized to their esters as well, using the same procedures.

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Method 8151A Spiking Standards

Question: In Method 8151A, should we spike field samples (matrix spikes) and the laboratory control samples (LCS) with the herbicide acids, or can we use the methyl esters?

Answer: You must use the form of the analyte that matches the form that is present in the samples, in other words, the parent acids, not the methyl esters. This is because the procedures in Method 8151A include both the derivatization and the determinative analysis. Therefore, spiking the QC aliquots with the methyl esters will not provide any information on the efficiency of the derivatization itself. If you are having problems with the recoveries of the acids, then spiking with the methyl esters and analyzing the samples again may be a useful diagnostic tool to see if the problem lies in the derivatization or only in the determinative steps. However, this would only be done infrequently, when the results for the parent acids suggested a problem.

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Alternative Internal Standards in Method 8260B

Question: When using SW-846, are we required to use the internal standards listed in Method 8260B or may we select other internal standards? We are using Method 8260B to analyze some samples for only the BTEX compounds and some of the internal standards are not relevant to our analytes of interest.

Answer: SW-846 does not require the use of the internal standards listed in the 8000 series methods for analyses. SW-846 only recommends the use of the internal standards listed in the methods. Normally, the internal standards listed in a particular method are those used to validate the procedure. However, all of the internal standards mentioned in Method 8260B may not be pertinent for a project where the analytes of interest are a smaller subset of the analytes listed in the method. When the list of target analytes is so short, the analyst may not need to use multiple internal standards if one or two will suffice to cover the range of retention times of the target analytes. As noted in Method 8000B, the target analytes should have relative retention times in the range of 0.80 to 1.20 compared to the internal standard. In the case of BTEX analyses, the analyst need only use those internal standards that are relevant to the BTEX compounds, e.g., having similar chemical and chromatographic behaviors and retention times.

The sample considerations apply to Method 8270C, or any other method that employs internal standards.

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Use of Average Response Factors vs. Daily Response Factors

Question: Should quantitation for Method 8260B (or Method 8270C) be done using the average response factor (RF) from the initial calibration or using an updated RF from the daily calibration check sample?

Answer: For SW-846 methods involving quantitative analyses, the average RF from the initial calibration should be used for quantitation. The daily calibration check standard is used for calibration verification, and it is fundamentally different from the approach called "continuing calibration" in the CLP organic Statement of Work (SOW) and some other programs. Use of the "continuing calibration" approach amounts to a daily single-point calibration and is not appropriate nor permitted in SW-846 chromatographic procedures for trace environmental analyses. Section of Method 8260B and Section 7.7 of Method 8000B contain the language that clarifies EPA's intent on this issue. There are a few screening procedures for organics in SW-846 that do utilize a single-point calibration for quantitation. However, these are clearly identified as screening procedures rather than truly quantitative methods.

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Relative Retention Time Units

Question: The GC/MS methods, 8260B and 8270C, refer to relative retention time (RRT) windows of 0.06 units. Please clarify exactly what that means. As I understand it, the RRT is a ratio of RT of target to RT of internal standard. How can that ratio have units?

Answer: Your understanding of relative retention time (RRT) is essentially correct. It is the ratio of the retention time of the analyte over the retention time of the internal standard.

Since both retention times will have the same units (whatever the lab chooses to use), those units can be viewed as cancelling one another out. Thus, RRT is often expressed as a unitless quantity.

Beginning with the earliest EPA GC/MS methods, EPA has provided an acceptance limit for the agreement of the RRT between the sample and the standard. The language has always read very much like what is in Sec. of Method 8260B, namely that:

"The relative retention time (RRT) of the sample component is within +/- 0.06 RRT units of the RRT of the standard component."

All that one is doing here is comparing the RRT of the peak in the sample analysis to the peak in the calibration standard. The RRTs must be within 0.06 of one another. Thus, for example, if the RRT of compound A in the calibration standard is 0.98, then the RRT of the peak in the sample analysis that you want to call compound A must be within the RRT range of 0.92 to 1.04. If the peak in the sample run is not within that window, then you may not call that peak compound A, since the RRT does not agree well enough.

We believe that the intent of the original wording of "0.06 RRT units" was to keep one from thinking that the acceptance limit of 0.06 is a simple 6% window. Otherwise, one might think that the two RRT values have to agree within 6% of the standard, which is a slightly narrower window (i.e., 6% of 0.98 is less than 0.06).

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Confirmation Column for Method 8310

Question: Method 8310, the HPLC method for PAHs, does not list a confirmation column. What column should I use to confirm my results from the primary column?

Answer: Work at the EPA laboratory in Region III (Annapolis, MD) suggests that the following columns may be appropriate for the analysis of PAHs by HPLC:

Standard analytical column - C-18 reversed-phase column, 3-5 m particle size, 100 - 250 mm x 4.6 mm ID, (Perkin-Elmer ChromSPHERE-3-PAH, Supelco Supelcosil LC-PAH, or equivalent).

Narrow-bore analytical column - C-18 reversed-phase column, 3-5 m particle size, 100 - 250 mm x 2.1 mm ID, (Supelco Supelcosil LC-PAH, or equivalent).

Confirmation column - CN reversed-phase column, 3-5 m particle size, 150 - 250 mm x 3.0 - 4.6 mm ID, (Supelco Supelcosil LC-PCN, Waters Nova-Pak CN HP, or equivalent).

However, other HPLC columns also may be available that will provide a sufficient change in elution order to be useful for confirmation.

Finally, you should consult Method 8000B for information on confirmation analyses, including when confirmations are needed and when they may not be needed.

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Confirmation Column for Method 8330

Question: We have been doing some method development with Method 8330. The method specifies confirmation by a cyano (CN) column. I have been able to get the CN column to resolve most of the target analytes, but I am not happy with the stability of the CN column itself. I have recently come across a proprietary C18 column with an amide linkage in the chain. This column, although it is technically classified as a C18, causes some significant elution order changes. Is it acceptable to use this other column for confirmations?

Answer: Method 8330 does not require that you use the cyano column for confirmations. First, confirmations may not even be necessary and this issue is discussed in detail in Method 8000B. The text in Sec. 4.1.2 of Method 8330 says "recommended columns" and that is all those two columns are - recommendations. You are free to use any columns you choose, provided that you demonstrate that the separation is appropriate for your intended application. That demonstration also applies to columns that are listed in methods - you need to show that what you do works. Therefore, if the other C18 column provides enough changes in the elution order, it may work very well for confirmations.

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